David Letterman unloads on fracking

"I'm not smart enough to understand it, but here is what I know about fracking," said David Letterman on his show on Wednesday.

"The greedy oil and gas companies of this country have decided that they can squeeze every last little ounce of oil and gas out of previously pumped wells by injecting the substrata of our planet with highly toxic, carcinogenic chemicals, which then seep into the aquifer and hence into the water supply of Americans."

New York state has a moratorium on hydraulic fracturing, while neighbouring Pennsylvania is exploiting reserves.